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Humidity/Transcript
Transcript 'Old Version' It is a hot summer day. Tim is in his driveway, washing a car. As he sponges soapy water on its hood, Moby sneaks up and squirts him with a water hose. TIM: Thanks. That actually felt good. Tim reads from a typed letter. TIM: Dear Tim & Moby, How can the air hold water? From, Jaqui. The air's kind of like a sponge. Moby pulls a wet sponge from a bucket and squeezes water from it. TIM: Water molecules can fit into the spaces between air molecules. Images of water droplets appear in the air around Tim. TIM: The amount of water vapor in the air is called humidity. When it's really humid outside, like today, it means that the atmosphere is filled with water. The water droplets around Tim increase in number and move closer together. TIM: Humidity changes from one day to the next, depending on the temperature. At 25 degrees Celsius, a cubic meter of air can hold 22 grams of water vapor. That same amount of air at 15 degrees Celsius can only hold 13 grams of water vapor. A bar graph illustrates relationships between specific temperatures and humidity levels, including the two relationships Tim describes. It indicates that the air can hold more water vapor as the temperature increases. TIM: So, warm air can hold a lot more moisture than cold air. Here's why. At cooler temperatures, air molecules move slowly and water molecules have time to join together, or condense. The molecules in warm air move faster. Because of all this activity, water molecules don't get the chance to condense. Animations compare slow-moving water molecules in cold air with fast-moving water molecules in warm air. The water molecules in cold air condense together to form larger molecules. MOBY: Beep. TIM: Yeah, then there's relative humidity. Relative humidity measures how much water the air is holding compared to the maximum amount it can hold at a given temperature. Our cubic meter of air at 25 degrees Celsius can hold a maximum of 22 grams of water vapor. When the air is holding as much water as it can, the relative humidity is 100 percent. An animation shows water molecules moving within a cubic meter of air. A thermometer represents a temperature of 25 degrees Celsius. Two filled glasses represent 22 grams of water. TIM: If the same cubic meter of air at the same temperature is holding 11 grams of water, then the relative humidity is 50 percent. That's one-half of the maximum water, so, 50 percent. The animation changes to one filled glass, representing 11 grams of water. MOBY: Beep. Moby points to a cup of water on top of the car. There are beads of water on the outside of the cup. TIM: Hey, your cup is sweating. The dew point is the temperature at which the air is fully saturated and condensation occurs. Any little drop in the air temperature means that the water condenses back into liquid or ice. An animation shows the mercury in a thermometer getting lower as water molecules condense around a glass of ice water. TIM: The colder temperature allows the water in the air to condense on the glass. Water drops form on the outside of the glass. TIM: So now you know when somebody says, "Boy, it sure is humid out," or, "I can't remember the last time it was this humid." Aargh! Moby extends his arm to hold a sponge over Tim's head. He squeezes water from the sponge, soaking Tim. TIM: All right. All right. Hey, give me the hose. I've got to rinse the car off. Moby squirts Tim with the water hose. TIM: Hey! Oh! Category:BrainPOP Transcripts Category:BrainPOP Science Transcripts